The present invention relates to a method and apparatus for preventing air conveyed by a moving material web from entering between a backing roll of a coater and said web.
The number of different defects in paper web coating increases rapidly with the increase of web speed in the coater. Consequently, when the web speed is increased, equipment and methods must be developed to be capable of maintaining a high coat quality, even at the high web speeds today requested above those used in the prior art. For example, coat application in a kissing roll coater encounters a defect type called skipping, which means mottled coating caused by uncoated blotches occurring on the coated web. In kiss roll coaters, the coating mix is applied by means of a kiss roll rotating in a coat mix tray onto the paper web passing over the backing roll and the coat is subsequently smoothed by means of a doctor blade. Tests performed indicate that one factor causing skipping is air entering between the web and the backing roll. The kiss roll is ordinarily rotated along with the web at a tangential speed of approx. 10-30% the speed of the web being coated. Alternatively, the kiss roll can be rotated counter to the web movement or at a different speed, whereby similar problems are met also in these cases. Air conveyed into the tapering space between the roll and the web causes equivalent complications in other application methods, too.
When the web speed is increased, the rotational speed of the kiss roll must be increased by an equivalent measure to avoid the occurrence of uncoated blotches. On the other hand, when the rotational speed of the kiss roll is increased, more splashing of the coat mix will occur, and eventually streaking of the coat, whereby the equipment and surroundings are soiled and paper quality deteriorated. Accordingly, a sufficiently low rotational speed of the kiss roll should be used that still can provide a coat of satisfactory quality. At maximum web speeds the rotational speed of the kiss roll unavoidably becomes so high as to cause splashing to a relatively high extent. Then, the rotational speed of the kiss roll need be reduced, which has not been possible in prior-art equipment due to the concomitant increase of coating defects.
Tests performed by the applicant have shown that the occurrence of uncoated blotches can be substantially lessened by reducing the amount of air entering between the coater backing roll and the web being coated. If the occurrence of coating defects can be prevented, a reduction of the kiss roll rotational speed in kiss roll coating methods becomes possible.
It is an object of the present invention to provide a method and an assembly suited for curtailing air entry between the coater backing roll and the web being coated.